1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus, and more particularly relates to a fixing device and an image forming apparatus including the fixing device for fixing an image on a recording medium.
2. Discussion of the Background
In a typical copying or printing process performed by an image forming apparatus, such as a copier, printer, facsimile machine, printing press and the like, an electrostatic latent image formed on an image bearing member or a photoconductive member is developed by conveying one or more developer materials such as toner particles to be transferred to the electrostatic latent image and is visualized as a toner image. The toner image is then transferred from the image bearing member onto a recording medium such as a recording sheet, and is conveyed to a fixing device. In the fixing device, the toner particles forming the toner image on the recording medium are subjected to a combination of application of heat and pressure to permanently affix the toner image to the recording sheet.
The fixing device may employ a heat roller fixing method in which a recording medium having an unfixed image thereon is conveyed while being sandwiched between a pair or rollers including a heat roller that has a heat source therein and a pressure roller capable of forming a fixing nip portion by facing the heat roller. The heat roller fixing method has used heat rollers including a material having a relatively large heat capacity. Such heat rollers may require a long period of time to raise the temperature thereof to a predetermined temperature, which results in a long standby period.
One way to achieve a lesser period of time to be raised to the predetermined temperature and a longer fixing nip portion is to use a deformable belt as a heat member spanned around a plurality of rollers and contacting toner. The deformable belt may have a thickness smaller than that of other heat members such as a roller so as to reduce a heat capacity thereof and thus reduce the period of rise time to the predetermined temperature. Thus, the flexibility of the belt allows a heat transfer area to be larger and longer.
Another way is to use a pair of endless belts which are spanned around respective rollers and face to each other while a recording medium having unfixed toner images on front and rear side surfaces thereof is sandwiched between the pair of endless belts. When fixing the toner images, respective fixing and pressure rollers internally contacting the pair of endless belts press each other so that the toner images can be permanently fixed to the recording medium.
Some toner particles heated and pressed may attach to the heat member, which may cause a hot offset. To prevent the hot offset, the heat member heretofore may have a surface of a layer including a fluorocarbon resin material or a surface overcoated with a releasing material such as silicone oil. Further, respective cooling mechanisms may be disposed opposite a fixing belt after a recording medium having toner images on front and rear side surfaces passes a fixing nip portion formed between a fixing roller and a pressure roller.
For recent demands for higher image resolution, the toner for visualizing the electrostatic latent image is studied to have further sphericity and a smaller particle diameter to form high definition images. As the toner prepared by pulverizing methods has a limit with respect to these properties, polymerized toners prepared by suspension polymerizing methods, emulsification polymerizing methods and dispersion polymerizing methods capable of conglobating the toner and making the toner have a small particle diameter are being used. Further, some toner may include a wax component to prevent the hot offset by itself.
To elongate the fixing nip portion (or length) formed between a pair of pressure rollers sandwiching a fixing belt, one of a pair of pressing roller presses the fixing belt toward the other of the pair of pressing rollers. The above-described application of pressure may form a depression or impression on the surface of the fixing belt along the periphery of other roller. Thus, a longer length for the fixing nip portion can be obtained.
The deformation of the fixing belt along the periphery of the other roller, however, may lose planarity of the fixing belt. Even though the deformation of the fixing belt may not significantly affect a recording medium having a relatively long length, it may produce a wrinkled surface or a deviated flap of a recording medium having a short length such as an envelope to lose the planarity at the fixing nip portion.
After passing the fixing nip portion, the recording medium may be removed from the fixing belt by a self stripping action due to a curvature of a roller. When an outer diameter of the roller is large, a stripping member such as a stripping finger disposed in the vicinity of the roller may be used to separate the recording medium from the fixing belt to compensate for a deficiency of curvature of the roller. When the stripping finger, however, is disposed to support the stripping action, the leading edge of the stripping finger may be held in contact with the periphery of the roller to pick up the recording medium. In this case, the leading edge of the stripping finger may rub or scrape the toner image formed on the recording medium and/or may wear or abrade the surface of the fixing belt, which may produce scratches on the recording medium and/or affect durability of the fixing belt due to abrasion. Toner particles may also be damaged when the fixing belt itself having scratches thereon rubs or scrapes the toner particles in the process of fusing or melting.
To reduce frictional resistance at the contact of the fixing belt and/or the stripping finger with the toner images and/or the toner particles, a releasing agent originally used for a prevention of the hot offset may be overcoated on the surface of the fixing belt. Some toner, however, may recently include a wax component to omit coating a lubricant onto belts and/or rollers to protect the belts and/or rollers. This makes it difficult to achieve an elimination of the above-described scratches.
When a belt-type fixing member is used, a period of rise time of the temperature can be reduced because the belt itself may need a lower heat capacity. The belt, however, is wound around supporting rollers including a fixing roller and a pressure roller. The fixing roller and the pressure roller are provided to form a fixing nip portion but are not provided with a heat roller. This structure may lose heat dissipated from the fixing belt to the fixing roller when the fixing device is not in operation. Further, the period of rise time to the predetermined temperature may be longer and/or an equilibrium state of the temperature may be unstable.